After the reaction and, if applicable, after the purification of secondary products formed during the reaction, polysaccharide derivatives are in a form which may be friable or lumpy or may resemble cotton wool. In this form, the polysaccharide derivatives still retain the structures determined by the raw materials. Thus, for example, cellulose ethers still exhibit the fibrous structure of the initial cellulose. These polysaccharide derivatives are thus unsuitable for use, for example, as products soluble in organic and/or aqueous media.
It is also necessary to establish specific particle-size distributions, bulk densities, degrees of drying and degrees of viscosity for the various fields of application.
In principle, virtually all polysaccharide derivatives have therefore to be compacted, ground and dried in order to be made suitable for use.
The cellulose derivatives rank among the industrially important polysaccharide derivatives. Their preparation, properties and applications are described, for example, in: Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, (1986), Volume A5, pages 461-488, VCH Verlagsgesellschaft, Weinheim, Methoden der organischen Chemie, 4th Edition (1987), Volume E20, Makromolekulare Stoffe, Part Volume 3, pages 2048-2076, Georg Thieme Verlag, Stuttgart.
DE-C 952 264 discloses a three-step process for converting moist, fibrous methyl celluloses into powder having a high rate of dissolution. Here the product, which contains 50 to 70 wt. % water, is first of all homogenised to form a plastic composition and cooled to 10° C. to 20° C., for which purpose a cooled screw press is used; the product is then ground using a hammer mill and dried in a forced-air dryer.
EP-A 0 049 815 (equivalent to U.S. Pat. No. 4,415,124) describes a two-step process for preparing micropowders from cellulose ethers or cellulose, wherein the products, which have a finely fibrous or woolly structure, are first of all converted into a brittle, compacted form and the material thus treated is subjected to a grinding process until a particle-size distribution of at least 90% under 0.125 mm. is attained. Oscillating mills or ball mills, preferably in cooled form, or pellet mills are used in the embrittling step and jet mills, pin mills or impact disk mills are used in the grinding step.
DE-A 30 32 778 mentions a two-step process for the continuous homogenisation of moist cellulose ethers by subjecting the moist cellulose ether to a cutting, impacting or shearing action, produced by circulating rotary bodies having casings with different profiles, at the same time optionally pelletising the comminuted cellulose ethers with addition of water, and subsequently drying the pellets obtained.
EP-B 0 370 447 describes a process for the non-destructive grinding and simultaneous drying of moist cellulose ethers, wherein a cellulose ether having an initial moisture content of 20 to 70 wt. % is conveyed by means of a transport gas and simultaneously comminuted by impact and friction and, by means of the energy of grinding, is dried to a residual moisture content of 1 to 10 wt. %.
EP-B 0 384 046 describes a process for preparing cellulose ether particles which involves the comminution of cellulose ethers having a viscosity of greater than 100 Pa·s—determined as 2% aqueous solution at 20° C., using an Ubbelodhe tube—to a particle size of 0.4 to 0.035 mm in a high-speed air-swept rotary impact mill.
JP-A 79 74855 describes a process for the preparation of pulverulent, soluble cellulose derivatives.
WO 96/0074 (equivalent to EP 0 767 811) mentions a process for comminuting cellulose ethers which involves the extrusion of a hydrated cellulose ether through orifices having a cross-sectional area of from 0.0075 mm2 to 1 mm2 (7.5×10−9 m2 to 1×10−6 m2) and cutting up the extrudate thus produced to the desired length.
EP-B 0 201 895 describes a process for preparing largely non-fibrous CMC material, characterised by treating the fibrous CMC with water and recovering the non-fibrous CMC through the addition of a liquid which does not dissolve the CMC. The known prior art processes are mostly multistep with a predryer or preliminary embrittlement or preliminary compaction. Furthermore, in all processes the chemical and/or thermal action on the macromolecules, particularly during the processing of highly viscous, highly substituted products, is invariably so high that during the grinding process the macromolecules are broken down in the sense that their chains are reduced in length, which is manifested in particular by a more or less extensive breakdown of the viscosity compared with that of the products initially used. Moreover, the surfaces of the products treated by means of preliminary embrittlement or preliminary drying steps become keratinised. In addition, common to all processes is the high expenditure of energy for grinding the polysaccharide derivatives after preliminary drying, preliminary embrittlement or preliminary compaction.